Systematic modification of the morphology of crystals can be achieved by tailoring additives which bind at a preselected crystal face, and thus inhibit growth in a predictable manner. Morphological changes associated with the growth of organic crystals in the presence of additives results from the high degree of specificity in the interaction of the foreign material with the different structured surfaces of the crystalline matrix. In general, when growth is inhibited in a direction perpendicular to a given crystal face, the area of this face increases relative to those of the other faces of the crystal. The additive is adsorbed only at those faces in which the part of the adsorbate that differs from that of the substrate points away from the crystal interior. Once adsorbed, the additive inhibits the regular deposition of oncoming layers of crystal substrate molecules, slowing down the growth perpendicular to that face and leading to a relative increase in its surface area.
Addadi et al. in Angew. Chem. Int. Ed. Engl., Vol. 24, pp. 466-485 (1985) disclose that when crystals of organic compounds are grown in the presence of growth inhibitors, there is a change in crystal morphology, and a stereochemical correlation exists between the modified morphology, crystal structure and the molecular structure of the inhibitor. This correlation has been exploited to resolve conglomerates, to engineer crystals with desired morphologies, to assign the absolute configuration of chiral molecules and crystals, and to design models for generation of optical activity. The dissolution of organic crystals in the presence of inhibitors is also described.
Shimon et al. in Nouveau Journal de Chemie, Vol. 10, No. 12, pp.723-737 (1986) disclose a general method for the controlled stereoselective etching of preselected faces of organic crystals. The method is based on a correlation between the molecular structure of the etchant and the organic crystals, and has been applied for sorting of enantiomorphs, assignment of configuration of chiral molecules, and assignment of the structure of polar crystals.
Japanese Patent Application 54115314 published Sep. 7, 1979 discloses recovery of adipic acid from a mixture of glutaric acid, succinic acid, and adipic acid by adding an inorganic potassium salt to the solution, separating the adipic acid by crystallization, and then purifying the adipic acid.
U.S. Pat. No. 3,818,081 issued Jun. 18, 1974 discloses separation of adipic acid from mixtures with glutaric and succinic acids by treatment with anhydrous ammonia.
The formation or presence of byproducts and impurities is an inherent and recurring problem in organic synthesis as well as manufacturing operations. Adipic acid synthesis results in a large number of impurities which are incorporated during crystallization, some of which are present in larger quantities and pose a quality problem. The most common ones are succinic, glutaric, and caproic acid, but others like fumaric, maleic, hydroxy caproic acid, and cyano valeric acid have been detected. The removal of such impurities by conventional means is often hindered by the similarity in solubility, chemical properties, and crystal structure between impurity and substrate.
The present invention provides a method of purification of adipic acid during crystallization through control of the accessibility of molecules to crystal growth sites thereby modifying crystal morphology. The present invention also provides a method of purification of adipic acid during crystallization by addition of caproic acid, or by addition of a surfactant which remains substantially unadsorbed.